Structural and theoretical studies of mono and di-insertion of symmetric alkynes into the Pd–C σ bond of cyclopalladated secondary (tert-butyl and ethyl) benzylamines

The reactivity of the dinuclear cyclopalladated complexes derived from secondary benzylamines, [Pd2{(C,N)eC6H4CH2NH(R)}2(m-X)2] [X ¼ Br, R ¼ t-Bu (1a); X ¼ Cl, R ¼ t-Bu (1b); X ¼ Cl, R ¼ Et (1c)] is reported. 1a reacted with dimethyl acetylenedicarboxylate (DMAD) to yield the mono-inserted complex [Pd2{(C,N)eC(CO2Me)]C(CO2Me)C6H4CH2NH(t-Bu)}2(m-Br)2] (2a). 1a, 1b and 1c reacted with (R0) C^C(R0) (R0¼ CO2Me, Ph) to give [Pd{(C,N) C(R0)]C(R0)C(R0)]C(R0)C6H4CH2NH(R)}X] [X ¼ Br, R ¼ t-Bu, R0¼ CO2Me (3a); X ¼ Cl, R ¼ t-Bu, R0¼ Ph (2b); X ¼ Cl, R ¼ Et, R0¼ Ph (2c)] through a double insertion of the alkyne into the PdeC s bond. 2c also reacts with Ag(CF3SO3) and pyridine to give mononuclear cationic complex [Pd{(C,N)eC(Ph)]C(Ph)C(Ph)]C(Ph)}C6H4CH2NH(Et)(py)]CF3SO3 (3c). The crystal structures of di-inserted complexes 2b, 2c and 3c have been determined by X-ray diffraction studies. Their molecular structures showed that the conformation of the C]C double bonds within the buta- dienyl group attached to the aromatic ring was transecis. Density functional theory (DFT) calculations also indicated that this arrangement is more stable than other possible conformations.